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Using a mid-infrared calibration of the Cepheid distance scale based onrecent observations at 3.6 um with the Spitzer Space Telescope, we haveobtained a new, high-accuracy calibration of the Hubble constant. We haveestablished the mid-IR zero point of the Leavitt Law (the CepheidPeriod-Luminosity relation) using time-averaged 3.6 um data for tenhigh-metallicity, Milky Way Cepheids having independently-measuredtrigonometric parallaxes. We have adopted the slope of the PL relation usingtime-averaged 3.6 um data for 80 long-period Large Magellanic Cloud (LMC)Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a newreddening-corrected distance to the LMC of 18.477 +/- 0.033 (systematic) mag.We re-examine the systematic uncertainties in H0, also taking into account newdata over the past decade. In combination with the new Spitzer calibration, thesystematic uncertainty in H0 over that obtained by the Hubble Space Telescope(HST) Key Project has decreased by over a factor of three. Applying the Spitzercalibration to the Key Project sample, we find a value of H0 = 74.3 with asystematic uncertainty of +/-2.1 (systematic) km/s/Mpc, corresponding to a 2.8%systematic uncertainty in the Hubble constant. This result, in combination withWMAP7 measurements of the cosmic microwave background anisotropies and assuminga flat universe, yields a value of the equation of state for dark energy, w0 =-1.09 +/- 0.10. Alternatively, relaxing the constraints on flatness and thenumbers of relativistic species, and combining our results with those of WMAP7,Type Ia supernovae and baryon acoustic oscillations yields w0 = -1.08 +/- 0.10and a value of N_eff = 4.13 +/- 0.67, mildly consistent with the existence of afourth neutrino species.

Carnegie Hubble Program: A Mid-Infrared Calibration of the Hubble Constant